Hammocks have been popularly used in various settings for relaxation and sleep. Their popularity likely stems from their unique design, relative simplicity, low cost, and, in some designs, portability. Hammocks are commonly used for resting outdoors without the mess and discomfort of lying directly on the ground. Hammocks can be found in various places from rural gardens and suburban yards to sandy beaches and camping sites under the shade of trees. They have even been used in the quarters of battleships. In general, hammocks require a frame or other support structure to which the ends of the hammock can be attached to suspend the hammock some height off the ground beneath it. In some instances, such as in some lawn and garden sets, the hammock is supported by a freestanding frame particularly adapted for use as a hammock support structure. In other uses, the hammock may simply be tied between two natural or pre-existing structures in a given setting, such as between two trees in a lawn, forest, or beach, for example.